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A sensor based on novel electroconducting polymer composite films, characterised with UV-vis spectroscopy, reveals rapid sensitivity to ammonia on a par with conventional PANI sensors as measured by optical absorption changes. The new composite, unlike PANI, is easy to regenerate. Aissam Airoudj, D. Debarnot, and F. Poncin-Epaillard of Le Mans University, working with B. Bêche of the Institute of Physiques in Rennes, France, have modified a polyaniline (PANI)/glycidyl ether of bisphenol A (SU-8) composite film and characterized it using UV-vis, Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Researchers are keenly pursuing electrically conducting polymers for a wide range of applications in display technology and for low-cost, low power sensors. Polymers have several distinct advantages over their inorganic counterparts. For instance, polymers are cheaper to produce than devices based on semiconductors. They can be processed much more readily using polymer chemistry without the need for sophisticated microlithographic techniques. Polymers films also have the advantage of flexibility, both literally and figuratively, in that they can be adapted to different types of chemical sensing based on electrical, optical or piezoelectrical transduction. Most of all, however, the wide variety of possible polymer types and compositions means that they can be produced to be sensitive to a range of chemical agents at room temperature and with high selectivity. "The sensing ability of electroconducting polymers is based on modulation of their doping level during redox or acid?base interactions with some gases," explain the researchers, "This effect results in an immediate alteration of the conductivity and the optical absorbance." The team has exploited these desirable characteristics in developing a sensor for the noxious gas ammonia. Fundamentally, the sensor is based on polyaniline (PANI), which shows marked electrical and optical variations in its properties when it comes into contact with oxidant?reductant chemicals. As such, it is seemingly an ideal sensor material for ammonia, and many research teams have investigated its potential. Unfortunately, PANI has poor solubility and its mechanical properties are not ideal for practical applications. Airoudj and colleagues have therefore turned to the idea of creating a PANI-containing composite material that would be less intractable than pure PANI. Other teams have recognised the potential of a PANI composite for various applications, although only one group had until now investigated the potential for gas sensing. Teams have produced composite films from a mixed polymer melt, and using chemical and electrochemical methods to incorporate additional materials into the polymer matrix. The reported PANI/PMMA for gas sensing showed slow response and long regeneration times. "To improve the characteristics of PANI sensors, we explored an epoxy resin as polymer matrix to elaborate conducting PANI composite," the team explains. The commercially available epoxy-based polymer, SU-8, has been used previously as a negative photoresist and has potential in micro-electro-optical-mechanical system (MEOMS). It is heat resistant, strong, and can be produced to varying thicknesses relatively easily. Now, the team has evaluated a PANI/SU-8 composite in an ammonia optical sensor using absorption spectroscopy to test its mettle. SU-8 alone is not sensitive to ammonia. The researchers found the composite material to be slightly less sensitive to ammonia gas, presumably due to the reduced number of adsorption sites on the film compared to pure PANI. However, this is offset significantly by the speed with which the composite responds to the presence of ammonia. The composite also regenerates much more rapidly, ready for a second round of testing, than pure PANI films, which can take up to half an hour for all ammonia to desorb from the surface. The team points out that only a small quantity of PANI need be present in the sensor film for it to be highly responsive to ammonia. This means that the film has all the mechanical and ease of processing advantages of SU-8 and is not restricted by the more stubborn physical properties of PANI. Related links:
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